fix(redis): use Upstash REST client

[RSO]

Summary

• Replace the shared Redis helper's long-lived TCP client with the @upstash/redis REST client to avoid serverless socket readiness and command-queue stalls.
• Support UPSTASH_REDIS_REST_URL/UPSTASH_REDIS_REST_TOKEN and Vercel KV KV_REST_API_URL/KV_REST_API_TOKEN env var pairs.
• Update Redis-backed experiment cache tests to key off REST credentials instead of REDIS_URL.

Verification

N/A; backend cache helper change was not manually exercised against live Redis REST credentials.

Visual Changes

N/A

Reviewer Notes

Operators must configure either UPSTASH_REDIS_REST_URL + UPSTASH_REDIS_REST_TOKEN or KV_REST_API_URL + KV_REST_API_TOKEN before deploying this change. The existing REDIS_URL path remains in use by chat state and was not removed.